Metadherin in Metastasis and Chemoresistance of Breast Cancer Summary: Most breast cancer patients succumb to recurrent tumors that spread to distant vital organs and resist currently available therapeutic regiments. Recent studies have identified Metadherin (MTDH) as a candidate metastasis gene that plays a dual role in promoting breast cancer metastasis and chemoresistance. Our proposed study is based on the following preliminary observations: 1) MTDH is located at the epicenter of 8q22, a genomic region that is frequently amplified in poor-prognosis breast cancer patients; 2) MTDH overexpression is detected in over 40% of breast tumor speciemens, and is strongly correlated with poor survival and higher risk of metastasis for breast cancer patients; 3) MTDH overexpression promotes metastastasis to lung and other organs in an experimental metastasis model. Conversely, inhibition of MTDH expression by RNA interference reduces lung metastasis; 4) MTDH promotes adhesion of breast tumor cells to vascular endothelial cells in vitro and increases metastatic seeding efficiency in animal metastasis assays; 5) MTDH enhances chemoresistance of breast cancer cells to a broad spectrum chemotherapeutic agents as well as other physiological stresses. MTDH-mediated chemoresistance is most pronouced when cancer cells are co-cultured with endothelial cells. Based on these preliminary observations, we postulate that the pro-metastasis and chemoresistance functions of MTDH are mediated through its interaction with unknown interacting protein(s) expressed by endothelial cells. Such interaction activates downstream signaling events and alters gene expression in both tumor and endothelial cells. Blocking the interaction between MTDH and its interacting proteins will inhibit metastasis and sensitize tumor cells to chemotherapies. The long-term goals of our research are to elucidate the molecular mechanisms of breast cancer metastasis and chemoresistance mediated by MTDH and explore the therapeutic applications of inhibiting MTDH function. To this end, we will use genetic and biochemical approaches to identify and characterize MTDH-interacting proteins that mediate tumor-endothelium interaction and explore the potential benefit of inhibiting such interaction (Aim1). We will define the molecular mechanism of MTDH induced chemoresistance and determine the therapeutic potential of MTDH targeting using experimental tumorigenesis and metastasis models (Aim2). Furthermore, we will generate transgenic and knockout mice for MTDH and test the effect of altered MTDH expression on tumorigenesis and metastasis in transgenic animals that are genetically prone to developing mammary tumors. Transgenic and knockout animals of MTDH will also allow us to directly address the physiological role of MTDH in development and mammary gland function (Aim3). The proposed study will identify key components of the molecular network that mediate the metastasis and chemoresistance functions of MTDH and provide new strategies to prevent or reduce chemoresistant metastasis of breast cancer. Metadherin in Metastasis and Chemoresistance of Breast Cancer. PUBLIC HEALTH RELEVANCE: Over 90% of breast cancer related deaths are due to inoperable relapsed tumors in distant vital organs that resist conventional chemotherapies. Our proposed studies will provide novel insights into the molecular mechanism of Metadherin in metastasis and chemoresistance. Furthermore, our research will facilitate the development of anti-metastasis therapeutics based on molecular targeting of Metadherin.